Magnetic Anisotropy Effects on Squareness Ratio of Nd–Fe–B Sintered Magnets With Different Coercivity

Abstract

To reduce the irreversible demagnetization of Nd–Fe–B sintered magnets at elevated temperatures, it is required to discover the method of improving the squareness ratio ( ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ ), where ${H} _{\mathrm{ k}}$ and ${H} _{\mathrm{ cJ}}$ are the knee field and coercivity, respectively. It is pointed out that the intrinsic nucleation field ( ${H} _{\mathrm{ N}}$ ) and instability field of uniaxial single-domain particles are determined the effects of the second anisotropy constant ( ${K} _{2}$ ) and oblique magnetic fields. ${H} _{\mathrm{ N}}$ is influenced by the misaligned grains which deteriorate ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ . Based on the results, we studied mainly the effects of ${K} _{2}$ to the first anisotropy constant ( ${K} _{1}$ ) on ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ for Nd–Fe–B sintered magnets at a temperature range from 298 to 473 K. It has also been examined the values of limiting condition ( $4{K} _{2}/{K}_{1} ) for preventing from the deterioration of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ . The samples used were Nd–Fe–B sintered magnets with ${H} _{\mathrm{ cJ}}$ of 0.99 and 2.73 MA/m. For a magnet with lower ${H} _{\mathrm{ cJ}}$ above approximately 400 K, the value of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ decreases rapidly, whereas ${K} _{2}/{K} _{1}$ increases. For a magnet with higher ${H} _{\mathrm{ cJ}}$ , ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ slightly decreases and ${K} _{2}/{K} _{1}$ gradually increases as the temperature increases. However, the temperature changes in the values of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ and ${K} _{2}/{K} _{1}$ for a magnet with the higher ${H} _{\mathrm{ cJ}}$ are considerably smaller than those of a magnet with the lower ${H} _{\mathrm{ cJ}}$ . The temperature dependence of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ and ${K} _{2}/{K} _{1}$ shows the opposite trends regardless of ${H} _{\mathrm{ cJ}}$ . The value of $4~{K} _{2}/{K} _{1}$ shows the opposite trend of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ also. According to a simulation of the torque curve, the direction of the easy axis was widely distributed in the case of a very large value for ${K} _{2}/{K} _{1}$ . The excessive ${K} _{2}/{K} _{1}$ contributes to spread out the region of easy axis, that is, it will hinder the uniaxial anisotropy which is essential for anisotropic permanent magnets. Most of these may be attributed to the effect of large ${K} _{1}$ of indispensable Dy for a very high ${H} _{\mathrm{ cJ}}$ magnet.